1. Field of the Invention
The present invention relates to a method for controlling an oxygen concentration sensor for use in an air/fuel ratio control system of an internal combustion engine.
2. Description of Backgroud Information
Systems for controlling the air/fuel ratio by feedback operation have been developed, in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor and the air/fuel ratio of mixture to be supplied to the engine is controlled using a feedback method to a target value in response to an output signal level from the 0.sub.2 sensor for the purpose of purifying of the exhaust gas and improving of the fuel economy.
As an oxygen concentration sensor for use in such an air/fuel ratio control system, there is a type which is capable of producing an output signal whose level is proportional to the oxygen concentration in the exhaust gas of the engine. For example, a critical current type oxygen concentration sensor which includes a flat oxygen ion conductive solid electrolyte member provided with a pair of electrodes on both of its main surfaces. The surface of one of the electrodes forms a part of a gas retaining chamber. This gas retaining chamber communicates with a flow of measuring gas such as the exhaust gas through a communication hole. This oxygen sensor is disclosed in Japanese Patent Application laid open No. 52-72286. In this oxygen concentration sensor, the oxygen ion conductive solid electrolyte member and the electrode pair serve as an oxygen pump element. If a current is supplied to the electrodes in such a manner that the electrodes facing the gas retaining chamber operates as a negative electrode, the oxygen component of the gas in the gas retaining chamber is ionized on the surface of the negative electrode of the oxygen pump element, and migrates through the inside of the oxygen pump element to the positive electrode where the oxygen ions are released from the surface in the form of the oxygen gas.
Under this condition, the magnitude of the critical current flowing through the electrodes becomes constant irrespective of the applied voltage but varies substantially in proportion to the oxygen concentration in the measuring gas. Therefore, by detecting the critical current value, the oxygen concentration in the measuring gas can be measured. However, if the air/fuel ratio is controlled by using this oxygen concentration sensor, the output signal from the oxygen concentration sensor becomes proportional to the oxygen concentration only when the air/fuel ratio of the mixture supplied to the engine is leaner than the stoichiometric air/fuel ratio. Therefore, an air/fuel ratio control operation using a target air/fuel ratio which is set in a rich range is not possible. As an example of oxygen concentration sensor capable of producing an output signal which is proportional to the oxygen concentration in the exhaust gas in both of the lean and rich ranges, the oxygen concentration sensor will include a pair of flat oxygen ion conductive solid electrolyte members each of which is provided with a pair of electrodes. The surface of one electrode of each solid electrolyte member forms a part of a gas retaining chamber which gas retaining chamber communicates with a flow of measuring gas through a communication hole. The surface of the other electrode of one of two solid electrolyte members faces an atmospheric chamber. This oxygen concentration sensor is disclosed in Japanese Patent Application laid open No. 59-192955. In this oxygen concentration sensor, one of two oxygen ion conductive solid electrolyte members and its electrode pair serve as an oxygen concentration ratio detection sensor cell element, and the other one of two oxygen ion conductive solid electrolyte members and its electrode pair serve as the oxygen pump element. By supplying a current so that the oxygen ions moves through the inside of the oxygen pump element toward the electrode located on the gas retaining chamber's side when the voltage generated across the electrodes of the oxygen concentration ratio detection sensor cell element is higher than a reference voltage, and so that the oxygen ions move through the inside of the oxygen pump element toward the electrode located on the other side of the gas retaining chamber when the voltage generated across the oxygen concentration ratio detection sensor cell element is equal to or lower than the reference voltage, the current value becomes proportional to the oxygen concentration both in the lean and rich regions.
In the case of this oxygen concentration proportional type oxygen concentration sensor, the temperature of the sensor must be sufficiently higher than (higher than 650.degree. C., for example) the temperature of the exhaust gas under a steady condition of the engine operation in order to obtain an oxygen concentration proportional type output signal characteristic. For this reason, a heater element is incorporated in the oxygen concentration sensor, to heat the oxygen pump element and sensor cell element. When the engine is started, the oxygen pump element and the sensor cell element must be heated by the heater element, to activate the oxygen concentration sensor quickly so that the feedback control of air/fuel ratio can begin.
However, the problem encountered in the conventional technique is that the oxygen pump element and the sensor cell element can be damaged by thermal shock if the oxygen pump element and the sensor cell element are heated rapidly by the supply of a heater current to the heater element during the time of a cold start of the engine.